The present invention relates to measuring relative fractions of liquid and vapor in mixed phase conductive fluids, such as occurs in boiling water, non-boiling turbulent flows, fluidized bed experiments, water-gas mixing analysis, nuclear plant cooling and other diverse applications.
The prior art includes a number of mechanical and electrical approaches to the problems. There have also been a number of attempts to measure the voidage or fractional vapor content in conductive fluids such as water by measuring the total capacitance of a section of the fluid, making use of the Clausius-Mosotti relationship to determine the fractional vapor content. Such an approach has the advantage of being easily calibrated since with reasonably simple sensor geometries, there is a linear relationship between average dielectric constant and liquid fraction. This technique works well in liquids of essentially zero conductivity but gives very poor or even completely erroneous results in liquids of even very low conductivity such as water.
The reason for this is that with currently attainable operating frequencies, the levels of capacitive currents are usually several orders of magnitude smaller than the resistive components. Even if the measuring system front end amplifiers can be kept from saturating on the large resistive current, in itself no small task, the presence of extremely small and unavoidable phase shifts in the signal circuits (fractions of a degree) will shift enough of the resistive component of current into the apparent capacitive domain to completely overwhelm the actual capacitive currents.
Another serious problem arises in the contact interface between the sensor and the liquid. Electrical contact is necessary, since otherwise the electric field will be developed across the insulator and not the fluid. Any measurements obtained with insulated sensors give more information about the quality of the insulation than about the fluid in the sensor region.
In view of these problems with capacitive measurements, recent work has been done in measuring the resistive component of the sensor current. This has the substantial advantage that sensor current is fairly large and easy to measure. However, there are three main difficulties associated with this approach. First, the calibration technique for determining voidage is more complex than in the capacitive case. It is not usually possible to arrive at a predicted relationship between measured conductivity and voidage. Instead, measurements must usually be made with various known levels of voidage and an empirical relationship derived.
Second, it is not possible to measure void content near 100%. If the resistive path across the sensor is broken, no measurements can be made. In many cases, this limitation is not a serious one.
Finally, conductive fluids such as water tend to undergo fairly dramatic changes in conductivity as a result of often uncontrollable circumstances such as absorption of carbon dioxide or other gases from the air and the possible leaching out of ionic salts from the containers and tubing. This problem can usually be overcome by making all measurements with respect to a reference sensor immersed with a sample of the test fluid. This sample must be representative of the fluid in the sensor and must contain no voidage.
It is an important object of the invention to provide vapor and liquid fraction measurement in a mixed flow affectively substantially free of the above problems.
It is a further object of the invention to provide a simple construction making a minimal disturbance on the system being measured consistent with the preceding objects.
It is a further object of the invention to screen out sources of spurious reading consistent with one or more of the preceding objects.
It is a further object of the invention to provide an economical device consistent with one or more of the preceding objects.